This invention relates to a non-blocking high capacity fine grain switch for use in a communications network, and to a communications network incorporating such a switch. The invention further relates to method of a controlling traffic in such a switch, and to a switch node incorporated in such a switch. The invention is particularly but not exclusively suitable for use in a telecommunications environment.
Photonic switches, such as can be implemented by micro-electromechanical (MEMS) devices, are an example of high bandwidth cross-connects which performing coarse granularity switching. Such switches enable the entire signal carried along an optical fiber to be switched to another optical fiber. For example, a thousand port photonic cross-connect receiving signals along each port at 1 Tbit/s can switch a traffic stream having a bandwidth of 1000 Tbit/s. However, the switching granularity of 1 Tbit/s is particularly coarse and has limited applications. Such switches do not support the switching of smaller bandwidth, or fine granularity, traffic.
The SONET (Synchronous Optical Network) standard defined by ANSI (American National Standards Institute) and the SDH (Synchronous Digital Hierarchy) standard defined by the ITU-T (International Telecommunications Union Telecommunications Standardization Sector) each specify a hierarchy of payload units. In SONET, the basic unit is an STS-1, offering a bandwidth of approximately 50 Mbit/s. In SDH, the basic unit is an AU-4, offering a bandwidth of approximately 150 Mbit/s.
Within the SDH and SONET standards, smaller bandwidth data structures can be incorporated and/or multiplexed into larger bandwidth structures. For example, within SDH channels with smaller bandwidths can be provided by, for example, VC-12 (approximately 2 Mbit/s) and VC-3 (approximately 50 Mbit/s). VC-12 and VC-3 channels are known as lower order data structures or lower order containers. Lower order containers are carried within higher order data structures or higher order containers such as VC-4. The VC-4's are then multiplexed to higher data rates. Similar data structure hierarchies exist within SONET. In both SONET and SDH environments therefore, the extraction and insertion of lower order data structures, or fine granularity traffic, to broader bandwidth traffic streams is known.
The capacity of fine granularity switches for switching traffic is generally much less than the capacity of coarse granularity switches such as photonic cross-connects. For example, a fine grain granularity SONET/SDH cross-connect is the SDH 4/1 cross-connect which has a switching granularity of one SDH VC-12. However, the size of the SDH 4/1 is limited and is unlikely to exceed 2000 STM-1 ports or a total bandwidth of 300 Gbit/s. Enhancing known fine granularity switches to have a capacity of 100's of Gbit/s generates bulky switches which can use up tens of racks of equipment. Fine granularity switches known in the art are unlikely to exceed 300 Gbit/s.
Simply combining several fine granularity switches together to form a larger capacity switch is not feasible as most such switches will be blocking. A combination of switches capable of providing both fine grain and coarse grain switching functions to form a large capacity switch such as FIG. 2 of the accompanying drawings does not provide a practical solution either as the switch formed can still be blocking in the manner described in more detail later.
It is highly desirable, particularly in the telecommunications environment, to provide a switch having a capacity large enough to be able to switch large bandwidth traffic yet which has the ability to switch fine grain (narrow bandwidth) traffic on demand. It is therefore advantageous if a switch can be provided which supports a switching operation over a plurality of different switching granularities, or different granularity switching levels. Such a switch can be conceptually perceived to have a layered structure. The central or core switching layer providing the coarsest granularity switching operation and each subsequent layer providing a finer granularity switching operation than the layer before. This layered architecture may or may not be reflected in the physical structure of the switch. It is extremely important that the architecture of any large bandwidth switch providing a fine granularity (narrow bandwidth) switching operation permits the switch to be suitably non-blocking.